Manufacture of cinematographic films with sound records



T. r. BAKER 1,955,045

MANUFACTURE OF CINEMATOGRAPHIG FILMS WITH SOUND RECORDS April 17, 1934.

2 Sheets-Sheet 1 Filed Nov. 18 1931 April 17, 1934. 1-, BAKER 1,955,045

MANUFACTURE OF CINEMATOGRAPHIC FILMS WITH SOUND RECORDS Filed Nov. 18, 1951 2 Sheets-Sheet 2 LAMP CURRENT M/LL/AMPERES.

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A 0 EA/gr 0F W506i x D J" 5 h LENGTH 0'' WEDGE lfivnvrlt Patented Apr. 17, 1934 PATNr OFFICE MANUFACTURE OF CINEMATOGRAPHIC FILMS WITH SOUND RECORDS Thomas Thorne Baker, Hatch End, England, as-

signor to Spicers Limited, London, England, a

British company Application November 18, 1931, Serial No. 575,917 In Great Britain December 24, 1930 4 Claims. (Cl. 179-1003) This invention consists of improvements in or relating to the manufacture of cinematograph 5 films with sound records include two types.

films with sound records.

The methods of manufacturing cinematograph In one type, hereinafter referred to as the variable density method, a source of light such as a potassium vapour lamp or a potassium and mercury vapor lamp is placed under the control of currents from a microphone (amplified it may be), so that the intensity of the light emitted at any instant by the lamp is a function of the microphone cur rent at that instant, and the variations due to the sound waves falling on the microphone are represented as to intensity and frequency by corresponding variations in the intensity and flickering of the light. The general custom is to project the light through a slit on to the sensitive film and the photographic image thus produced has variations of density corresponding to the variations in the microphone current. The photographic image in this case extends the full width of the sound track on the film.

In another type, generally known as the variable area method, the sound-sensitive device is of the nature of a metal string electromagnetically controlled in accordance with the microphone currents (i. e. a string galvanometer) and the movements of the string obturate to a greater or less extent a beam of light directed on to the film. In this case the variations in width of the photographic image obtained on the film correspond to the variations in the microphone current.

The present invention relates to a modification of the variable density method of producing a photographic sound record on a film in which a screen of opacity varying along its length is so disposed across the light path from a sound-sensitive light source that the length of the lightband falling on the film at any instant is a function of the intensity of the light-source at that instant. generally in the prior British Patent 279,826 and in the British Illustrated Ofiicial Journal (Patents) for March 31, 1921, page 604, specification 157,441.

By the term sound-sensitive light source is to be understood an electric discharge device Which,'when connected (preferably through an amplifier) to a microphone or equivalent device (e. g. an electric pick up) for transforming fluctuations in sound into fluctuations in electric current, will emit a beam of light corresponding at each instant in characteristics to the characteris- The method is illustrated and described tics to the sounds being recorded by the microphone at that instant.

One object of the present invention is to sesure on a film a photographic sound record oii consistent exactitude, not injuriously affected by extraneous influence such as the characteristics of a photographic emulsion (e. g. irregularities in the density-log. exposure curve).

The particular sound-sensitive light source (potassium vapour lamp or the like) employedmight not have a straight line characteristic and it is considered an important object of this invention to obtain photographic sound records which are uniformly exact and are not injuriously affected by such irregularities in the characteristics of the sound-sensitive device.

Again the microphone might not have straight line characteristics (viz, sound intensity-current amplitude characteristic).

Again the amplifier between the microphone and the light source might not have a straight line characteristic.

For sound recording, photographic emulsions of special properties are normally required. The

emulsion should in the first place have a fine grain. In the case of the above described variable density system of sound recording the emulsion should have as long a scale of gradation as possible. In the case of the variable area system the essential requirement is that the emulsion should give absolute opacity, i. e. it should give a dead black image.

It is one of the principal objects of this invention to provide means whereby the sound record may be made on a normal photographic emulsion,

i. e. one which does not possess the special characteristics which have hitherto been necessary for sound recording, or may be modified on any emulsion to suit the subsequent printing emulsion.

The invention provides a method of the type described for the production of a sound record on a film which is characterized by the fact that the opacity of the screen is so varied along its length as to compensate for variations in the characteristics of those instrumentalities which influence the record (e. g. the characteristics of the emulsion, of the sound-sensitive device, of the microphone, or of the amplifier).

The sound sensitive light source may be constituted by a potassium vapour lamp. Alternatively, the sound-sensitive light source may be constituted by a gas discharge lamp having a light emitting cathode of the kind used in television receivers. In this case the lamp preferably has a rectangular cathode the dimensions of which are preferably about 2 inches by 1 inch, and the screen is constituted by a neutral grey wedge arranged with its long side parallel to the long sides of the rectangular cathode.

The lamps employed for television reception are customarily filled with neon, but a filling of a mixture of argon and mercury vapour or other quick response vapour may be employed instead of neon.

Such lamps respond to the variations in'the microphone currents as quicklyas the potassium vapour lamp hitherto employed in the recording of sound by the variable density process, and have the great advantage that'they emit a far greater area of light, thus being eminently suited to the use of a neutral grey wedge.

The invention includesapparatus for use in the production of photographic sound records which comprises a microphone, a sound-sensitive light source (e. g. a potassium vapour lamp or a gas .,discharge lamp having a light-emitting cathode of the kind used in television receivers) connected to the microphone (preferably through an ampli- -fier) and adapted to emit a beam of light corresponding in characteristics at each instant to the sound being recorded by the microphone at that instant, and an optical system arranged to focus an image of the screen on to a moving strip of film, in combination with a neutral grey wedge (e. g. about 2 inches long and inch wide), the opacity of which isvaried along its length in such manner as to compensate for undesirable characteristics of any or all of the instr-urnentalities which influence the record, such for example as the emulsion, the microphone, the amplifier, or the electric discharge device. Preferably, the light from thedisoharge device is distributed by a lens over the wedge and the image of the wedge is concentrated on the film through a narrow slit (e. g. th inch long. and 1/200th inch wide) The density range of the wedge may conveniently be from 0-3.

. Normally in taking sound films the sound rec- 0rd and the picture record are taken on separate films in separate cameras, but for reproduction the sound record and the picture record mustbe printed on the same film. Thus even though an emulsion suitable for sound-recordirr may have been selected for the original sound record, difficulties generally arise later when the record is printed on the sound-track of the film used for reproduction, as the emulsion used for thisfilm cannot in general have characteristics which-will give satisfactory results both for picture-record- .ing and for sound-recording. This dirficulty is especially noteworthy in the case of a film having a colour-sensitive emulsion, e. g. a film of the .kind. in whichexposure is effected through a multicolour screen associated with the film, as in this case the emulsion is one which is quite unsuitable for soundrecording.

According to a further feature of the invention the opacity of the screen through which the sound-record is taken may be so varied along its length as to compensate for thecharacteristics not only of the emulsion on which the soundrecord is recorded, but also of the emulsion (for example a colour-sensitive emulsion) of a film .used for the simultaneous reproduction .of the sound-record and a picture record on which the sound-record is subsequently printed. Thus even though the sound-record is being recorded on an emulsion having characteristics suitable .for

, sound-recording the recording maybe purposely modified so as to give a recordwhich, though erroneous for normal printing stock will, when printed on a colour sensitive copying emulsion, give a correct record. This is due to the fact that the deficiencies of the copying emulsion for sound-recording have been compensated for by the equal and opposite deficiencies purposely introduced during the taking of the original soundrecord.

the sounds. The microphone current is amplified by the valve 8, and led to the sound-sensitive light source 9. In the drawings, a potassium vapour lamp is shown in the light source; Instead of this, however, a gas-discharge lamp having a lightemitting cathode of the type used in television receivers may be employed, the cathode in'this case-being rectangular and having an area-of about 2 inches by 1 inch. Such lamps respond'as? quickly to variations in the microphone current as do potassium vapour lampsgandthey havethe great advantage that they emit a far greater area of light and are therefore especially suitable for use with neutral grey wedges in the manner described herein.

The electrical circuit employed 'for coupling the microphone 7 with the lamp 9 is shown in Figure 2. 10 is the microphone transformer, 11 a battery in series with-the microphone, 30 a grid bias battery for the valve 8, 12- a milliameter,

and .13 a high tension battery of about 700 volts.

For the sake of simplicity the means'for heating .thefilament of the valve 8 are omitted from-Figure 2, as also are the early stages of amplification.- The lamp 9. is-placed at the'focus of a short focus lens 16 (having for example a focal length of 3. inches), so that a substantially parallel beam of light is thrown on the neutral grey wedge 17. 18 is a micrometer adjustment for the wedge. A convenient density range for the wedge is from 0-3. The wedge conveniently forms a window 2 inches long and inch Wide. In the casewhere a lamp of the. type described above having a rectangular cathode is used, the Wedge is arranged with its longside parallel to that of the cathode. The rays passing through the wedge are collected and concentrated by a lens system comprisinga correcting lens 19 and an achromatic focussing combination 20, so that the image of the wedge is concentrated through the slit 21 on to the'film 22; the length of the image being transverse to the length of the film. The slit 21 is conveniently about %th inch long and 1/200th inch wide. The film is wound continuously from the feed spool 23 to the take-up spool 24 past the slit 21.

The' actual length of the photographic image obtained on the film is a greater or smaller fraction of th inch. Inother words, asis customary with a neutral grey'wedge, if the light from the source is very intense the lengthof the-band passing through the wedge will be a maximum, and as the intensity of the light from the source diminishes, the length of the-band-focussedon:

the film correspondingly diminishes. The photographic image is therefore of a character sharply varying in length and serrated by the sound frequencies.

In the case of a sound track on a colour film, particularly a film of the kind in which the emulsion is exposed through a multi-colour screen, it is not only necessary to use a panchromatic emulsion, but it is also advisable in some colour cinematograph transparencies to use a very low proportion of silver salts per unit area. Such emulsions are not normally suitable for producing a sound track. The use of the screen, e. g. the neutral grey Wedge above referred to, in the first place affords the opportunity of producing on the film a sound track which can be satisfactorily used for sound reproduction by the use of standard appliances, and in the second place, by a suitable variation in the opacity of the screen or wedge it is possible to compensate with exactness for any variation in the characteristics of the emulsion.

The method by which the wedge may be used to correct undesirable characteristics in the lamp amplifier, emulsion etc. will be understood by reference to Figures 3-6 of the drawings.

Figure 3 is a curve between the volts across the lamp and the lamp current for a lamp which has not got a straight line characteristic.

Figure 4 is a curve showing the variation in density across the length of the wedge.

Ac shows the straight-line density curve for an ordinary neutral grey wedge. In order to compensate for the curvature at the upper end of the characteristic curve of the lamp, the opacity of the wedge is varied to conform to the curve ABD. This can be done by known photographic or mechanical methods which do not, per se, form a feature of the present invention.

Figure 5 shows the characteristic curve of an emulsion tending to give too dense high lights. Figure 6 shows how the opacity of the wedge may be varied to compensate for this undesirable characteristic of the emulsion. In Figure 6, AB represents the normal absorption curve of the wedge, ACE is the curve of a wedge which has been modified to counteract the undesirable characteristics of the emulsion, the characteristic curve of which is shown in Figure 5. ACD is the absorption curve of a wedge, the opacity of which has been modified to compensate both for the emulsion curve shown in Figure 5 and the lamp curve shown in Figure 3.

Similarly, the opacity of the wedge may be varied to compensate for variations in the characteristics of the amplifier and other instrumentalities which influence the sound records.

The invention includes a photographic film bearing a sound record produced by the method described and claimed herein.

I claim:-

1. Apparatus for use in the production of photographic sound records comprising in combination a gas discharge lamp having a rectangular light-emitting cathode, a microphone to record the sounds operatively coupled to the lamp, a neutral grey wedge having a density ratio of from 0 to 3, arranged with its long side parallel to the long side of the aforesaid cathode, and having an opacity which is varied along its length in such manner as to compensate for undesirable characteristics of the lamp and the emulsion on which the record is to be photographed, a narrow slit arranged parallel to the wedge, an optical system arranged to concentrate the light transmitted by the wedge through the slit onto the film, and means to move the film continuously past the slit in close proximity thereto.

2. In apparatus for use in the production of photographic sound records, the combination with a photographic film, of a sound-sensitive light source, means to cause the light emitted by said source to fluctuate in correspondence with fluctuations in the sound to be recorded, a screen interposed between said light source and said film, the opacity of said screen increasing generally in one direction transverse to said film but varying from a uniformly increasing opacity, the variation in opacity of the screen as measured transversely of the film being such as to compensate for deviation from the correct response in the response of the emulsion to different light intensities, and means confining the image of the screen on the film to a narrow slit extending transversely of the film.

3. In apparatus for use in the production of photographic sound records, the combination with a photographic film, of a sound-sensitive light source, means to cause the light emitted by said source to fluctuate in correspondence with fluctuations in the sound to be recorded, a screen interposed between said light source and said film, the opacity of said screen increasing generally in one direction transverse to said film but varying from a uniformly increasing opacity, the variation in opacity of the screen as measured transversely of the film being such as to compensate for deviation from the correct response in the response of the emulsion of a film on which the sound may be recorded to different light intensities, and means confining the image of the screen on the film to a narrow slit extending transversely of the film.

4. In apparatus for use in the production of photographic sound records, the combination with a photographic film, of a sound-sensitive light source, means to cause the light emitted by said source to fluctuate in correspondence with fluctuations in the sound to be recorded, a screen interposed between said light source and said film, the opacity of said screen increasing generally in one direction transverse to said film but varying from a uniformly increasing opacity, the variation in opacity of the screen as measured transversely of the film being such as to compensate for deviation from the correct response in the response of the light sensitive source to the sound fluctuations.

' THOMAS TI-IORNE BAKER. 

